A storage system typically includes a storage device including memory such as semiconductor memory, and a host that uses the storage device for storing data. Conventionally, such a storage device includes a controller, and the controller manages data storage in the memory so as to reliably store data received from the host. On the other hand, it may be desirable for the host to more actively participate in the management of the data storage in the memory. This is because the controller may be already carrying out an operation on its own initiative when the host instructs the controller to perform a certain operation, and the instructed operation may not be timely performed by the controller until the controller completes the already-started operation.